Is there a way to define the docstring for a python object that defines __call__?

What you want is not doable no. The help() information (and inspect information, etc.) will show it as a class with a __call__ method, not a function.

If the object you produce doesn't have to support anything other than calling, you could use a closure to return an actual function:

def produce_callable(*args, **kw):
    instance = YourClass(*args, **kw)
    def callable_function(*args, **kw):
        """Docstring here"""
        return instance(*args, **kw)
    return callable_function

So rather than return your custom class instance, you return a wrapper function that'll call the class instance. help() information will be given for that function.

I used the *args and **kw wildcard arguments in the callable_function() signature, but if you know what arguments are accepted by the __call__ method, you can hard-code those in the function you produce.